KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

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Summary

CVECVE-2025-21839
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2025-03-07 09:15:16 UTC
Updated2026-07-30 06:21:40 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop Move the conditional loading of hardware DR6 with the guest's DR6 value out of the core .vcpu_run() loop to fix a bug where KVM can load hardware with a stale vcpu->arch.dr6. When the guest accesses a DR and host userspace isn't debugging the guest, KVM disables DR interception and loads the guest's values into hardware on VM-Enter and saves them on VM-Exit. This allows the guest to access DRs at will, e.g. so that a sequence of DR accesses to configure a breakpoint only generates one VM-Exit. For DR0-DR3, the logic/behavior is identical between VMX and SVM, and also identical between KVM_DEBUGREG_BP_ENABLED (userspace debugging the guest) and KVM_DEBUGREG_WONT_EXIT (guest using DRs), and so KVM handles loading DR0-DR3 in common code, _outside_ of the core kvm_x86_ops.vcpu_run() loop. But for DR6, the guest's value doesn't need to be loaded into hardware for KVM_DEBUGREG_BP_ENABLED, and SVM provides a dedicated VMCB field whereas VMX requires software to manually load the guest value, and so loading the guest's value into DR6 is handled by {svm,vmx}_vcpu_run(), i.e. is done _inside_ the core run loop. Unfortunately, saving the guest values on VM-Exit is initiated by common x86, again outside of the core run loop. If the guest modifies DR6 (in hardware, when DR interception is disabled), and then the next VM-Exit is a fastpath VM-Exit, KVM will reload hardware DR6 with vcpu->arch.dr6 and clobber the guest's actual value. The bug shows up primarily with nested VMX because KVM handles the VMX preemption timer in the fastpath, and the window between hardware DR6 being modified (in guest context) and DR6 being read by guest software is orders of magnitude larger in a nested setup. E.g. in non-nested, the VMX preemption timer would need to fire precisely between #DB injection and the #DB handler's read of DR6, whereas with a KVM-on-KVM setup, the window where hardware DR6 is "dirty" extends all the way from L1 writing DR6 to VMRESUME (in L1). L1's view: ========== <L1 disables DR interception> CPU 0/KVM-7289 [023] d.... 2925.640961: kvm_entry: vcpu 0 A: L1 Writes DR6 CPU 0/KVM-7289 [023] d.... 2925.640963: <hack>: Set DRs, DR6 = 0xffff0ff1 B: CPU 0/KVM-7289 [023] d.... 2925.640967: kvm_exit: vcpu 0 reason EXTERNAL_INTERRUPT intr_info 0x800000ec D: L1 reads DR6, arch.dr6 = 0 CPU 0/KVM-7289 [023] d.... 2925.640969: <hack>: Sync DRs, DR6 = 0xffff0ff0 CPU 0/KVM-7289 [023] d.... 2925.640976: kvm_entry: vcpu 0 L2 reads DR6, L1 disables DR interception CPU 0/KVM-7289 [023] d.... 2925.640980: kvm_exit: vcpu 0 reason DR_ACCESS info1 0x0000000000000216 CPU 0/KVM-7289 [023] d.... 2925.640983: kvm_entry: vcpu 0 CPU 0/KVM-7289 [023] d.... 2925.640983: <hack>: Set DRs, DR6 = 0xffff0ff0 L2 detects failure CPU 0/KVM-7289 [023] d.... 2925.640987: kvm_exit: vcpu 0 reason HLT L1 reads DR6 (confirms failure) CPU 0/KVM-7289 [023] d.... 2925.640990: <hack>: Sync DRs, DR6 = 0xffff0ff0 L0's view: ========== L2 reads DR6, arch.dr6 = 0 CPU 23/KVM-5046 [001] d.... 3410.005610: kvm_exit: vcpu 23 reason DR_ACCESS info1 0x0000000000000216 CPU 23/KVM-5046 [001] ..... 3410.005610: kvm_nested_vmexit: vcpu 23 reason DR_ACCESS info1 0x0000000000000216 L2 => L1 nested VM-Exit CPU 23/KVM-5046 [001] ..... 3410.005610: kvm_nested_vmexit_inject: reason: DR_ACCESS ext_inf1: 0x0000000000000216 CPU 23/KVM-5046 [001] d.... 3410.005610: kvm_entry: vcpu 23 CPU 23/KVM-5046 [001] d.... 3410.005611: kvm_exit: vcpu 23 reason VMREAD CPU 23/KVM-5046 [001] d.... 3410.005611: kvm_entry: vcpu 23 CPU 23/KVM-5046 [001] d.... 3410. ---truncated---

Risk And Classification

Primary CVSS: v3.1 5.5 MEDIUM from [email protected]

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Problem Types: NVD-CWE-noinfo

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary5.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
3.1416baaa9-dc9f-4396-8d5f-8c081fb06d67Secondary7.3HIGHCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:H
3.1CNADECLARED7.3HIGHCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:H

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Operating System Linux Linux Kernel All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected d67668e9dd76d98136048935723947156737932b 9efb2b99b96c86664bbdbdd2cdb354ac9627eb20 git Not specified
CNA Linux Linux affected d67668e9dd76d98136048935723947156737932b 93eeb6df1605b3a24f38afdba7ab903ba6b64133 git Not specified
CNA Linux Linux affected d67668e9dd76d98136048935723947156737932b a1723e9c53fe6431415be19302a56543daf503f5 git Not specified
CNA Linux Linux affected d67668e9dd76d98136048935723947156737932b 4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55 git Not specified
CNA Linux Linux affected d67668e9dd76d98136048935723947156737932b d456de38d9eb753a4e9fde053c18d4ef8e485339 git Not specified
CNA Linux Linux affected d67668e9dd76d98136048935723947156737932b c2fee09fc167c74a64adb08656cb993ea475197e git Not specified
CNA Linux Linux affected 5.7 Not specified
CNA Linux Linux unaffected 5.7 semver Not specified
CNA Linux Linux unaffected 5.15.182 5.15.* semver Not specified
CNA Linux Linux unaffected 6.1.138 6.1.* semver Not specified
CNA Linux Linux unaffected 6.6.90 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.16 6.12.* semver Not specified
CNA Linux Linux unaffected 6.13.4 6.13.* semver Not specified
CNA Linux Linux unaffected 6.14 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
lists.debian.org/debian-lts-announce/2025/08/msg00010.html af854a3a-2127-422b-91ae-364da2661108 lists.debian.org
git.kernel.org/stable/c/4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/d456de38d9eb753a4e9fde053c18d4ef8e485339 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/a1723e9c53fe6431415be19302a56543daf503f5 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/9efb2b99b96c86664bbdbdd2cdb354ac9627eb20 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/c2fee09fc167c74a64adb08656cb993ea475197e 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/93eeb6df1605b3a24f38afdba7ab903ba6b64133 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
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